FILTRATION AND BACKWASHING A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332
Particle Removal • Improve taste, appearance • Sorbed metals and pesticides • Pathogens: bacteria, viruses, protozoa
Organic Removal in Biofiltration • Prevent biofouling of distribution system • Remove DBP precursors
Multiple-Barrier Concept chemical addition direct filtration filtration watershed protection sedimentation disinfection raw water distribution system screen coagulation flocculation waste sludge backwashrecycle waste sludge
Fundamental and Microscopic View 1. Filtration: • Attachment • Detachment 2. Backwashing: • Detachment
Mechanisms of Filtration particle, dp transport fluid streamline attachment collector, dc detachment
History of Filtration Theory(1) Phenomenological - Macroscopic View Basic Equations: Ives:
Trajectory Theory dp Viruses 0.01 -0.025 mm Bacteria 0.2 -1mm Cryptosporidium 3 - 5mm Giardia 6 - 10mm dc dc dc Diffusion dp < 1 mm Sedimentation dp > 1 mm Interception
History of Filtration Theory (2) Trajectory Analysis - Microscopic View
Detachment - Macroscopic View Mintz: Ginn et al.:
Filter Effluent Quality Filter Ripening Outlet Backwash remnants TB above media in media TM Effluent Turbidity Function of influent Clean back-wash Media Strainer TU Filter breakthrough TU TM TB TR Time
Conceptual Model of Filtration Attachment(+) Filter coefficient () 0 (-) Detachment Filter Ripening Effective Filtration Turbidity Breakthrough Wormhole Flow Time
Question: Why is it easier to remove alum or clay particles in contrast to polymer coated particles or micro-organisms during backwash?
Sphere - Flat Plate Interactions (1) Van der Waals Force: a z Electrostatic Double Layer Force:
Detachment During Backwashing Hydrodynamic Forces > Adhesive Forces 1. Spherical Particles - pH and Ionic Strength 2. Non-spherical Particles - Ionic Strength • Kaolinite Platelets
Backwashing Filters • Weakness of fluidization backwash • Improvement due to surface wash • Collapse-pulsing air scour The best for cleaning
Theory for Collapse-Pulsing a, b = coefficients for a given media Qa = air flow rate Percentage of minimum fluidization water flow
Biofiltration • Ozonation • Microbial counts in effluent • Head loss • Effect of biocides • Particle removal
Biological Filtration and Backwashing • Precursor Removal • Minimize DBP’s • Effect of Hydrophobicity
Bacterial Adhesion Energy barrier Repulsion Potential Energy of Interaction Distance Secondary Attraction minimum Release of extracellular polymeric substances at secondary minimum Primary minimum
Backwashing Biofilters • Collapse-pulsing air scour • Cleans better • No deleterious effect • Chlorinated backwash reduces TOC removal over time • Chloraminated backwash less than 2.0 mg/L may be used
Pathogenic Protozoa • Low infective doses • Resistant to chlorine disinfection • Analytical techniques
Outbreaks of Cryptosporidiosis • Surface and groundwater sources • Runoff • Sewage spills • Coagulation • Filtration • rate changes • Backwash recycle • Contaminated distribution system
Particle Counts • Continuous on-line monitoring • Low operating costs • High sensitivity • Detachment of aggregates
Cyst Removal vs Particle Removal Nieminski and Ongerth (1995)
Minimizing Risk of Outbreaks • Optimal destabilization of particles • Filter-to-waste • Coagulants in backwash • Slow-start filtration • Minimizing flow rate changes in dirty filters • Treatment of backwash water • Filter effluent turbidity < 0.1 NTU
Concluding Statement In the multiple-barrier concept, filtration is the “great” barrier to particles, parasites and organics.
Summary and Conclusions • Importance of particle destabilization • Micromechanical force model • Biofiltration for organics removal • Effectiveness of collapse-pulsing air scour • Multiple-barrier concept
References • Amirtharajah, A., “Some Theoretical and Conceptual Views of Filtration,” JAWWA, Vol. 80, No. 12, 36-46, Dec. 1988. • Amirtharajah, A., “Optimum Backwashing of Filters with Air Scour - A Review,” Water Sci. and Tech., Vol. 27, No. 10, 195-211, 1993. • Ahmad, R. et al., “Effects of Backwashing on Biological Filters,” JAWWA, Vol. 90, No. 12, 62-73, Dec. 1998.
Acknowledgments This paper includes the work of several former students at Georgia Tech: M.S. students T.M. Ginn, L. Zeng and X. Wang and Ph.D students, Drs. P. Raveendran, R. Ahmad, K.E. Dennett and T. Mahmood. They were not only students but teachers too! Their work is acknowledged with gratitude.